Okay, a few notes and a reality check (from my part of the world anyway):
If a horsefly or deerfly bites you, it will hurt. Their mandibles are rather like those giant tree-mover trucks -- they just reach in and carve out a hunk of meat.
Avoiding bites is the cure. Their primary prey is large animals (like horses, and deer) who can't protect every square inch of their bodies. This also includes large dogs: when I had Great Danes, they would come roaring up to me for help, because the little buggers would bite them right in the depression where the backbone and tail meet -- the one place dogs cannot reach.
These pests are slow and noisy; you can hardly miss the fact that they are hovering around you. Cover up and apply DEET. Also note that they take a lot of killing; if you knock one to the ground, finish it with full vigour, or it will be back.
Note: if you are backpacking, avoid heavily-travelled horse trails. The pests inhabit the same terrain, for obvious reasons.
I have never heard of a person (in my part of the world) contracting a disease from these. But no doubt this varies with geography, so know your (local) onions.